This invention relates to compensating for errors in the measurement of the kinematic properties of an electric power steering motor for an automobile.
An electric power steering apparatus for a motor vehicle or automobile typically uses an electric motor to assist an operator in applying the necessary torque required to steer the vehicle. When the vehicle is steered with a steering wheel operably connected to a set of road wheels, a sensor in the electric power steering apparatus detects the angular position and/or velocity of the motor. A signal is then sent from the sensor to an electric controller. The electric controller controls the magnitude and direction of the steering assist provided by the electric motor. The electric motor drives a reducing gear, typically a worm gear engaging a worm wheel, that lessens the torque required to turn the steering wheel. Electric steering systems often utilize a motor with a digital encoder for a position sensor.
Electric power steering systems for motor vehicles may require motor velocity as part of the control law. In order to estimate the motor velocity, either a time derivative of motor position or direct measurement of motor velocity must be performed. In order to minimize the cost and complexity of the motor sensing subsystem, a relatively low resolution position sensor is desireable. A technology used for this motor sensing function includes a magnetic target with corresponding magnetic sensors (i.e., Hall Sensors). One problem of utilizing this approach is the potential variability of the output signals. A second problem with this approach is that at low motor velocities, the low resolution of the sensor does not provide sufficient angular input resolution for a time base derivative where the position change is variable and the time base is constant. Therefore, the time base derivative must be based on a constant position change and a variable time base. This causes the derivative to be very sensitive to errors in the angular position signal.
A method of compensating for errors in the measurement of the kinematic properties of an electric power steering motor for an automobile is disclosed. The method comprises generating a set of M-ary signals; measuring the pulse width of the set of M-ary signals; and measuring the time between the edges of the set of M-ary signals. The motor position signal can be used to estimate the angular velocity of the motor, by determining the change in angular position of the motor divided by the change in time.